In a multiple-input multiple-output (MIMO) technology, resources in spatial dimension are used, so that a signal may spatially obtain array gains, multiplexing and diversity gains, and interference cancellation gains without increasing a system bandwidth, thereby exponentially improving a capacity and spectral efficiency of a communications system. For example, in a Long Term Evolution (LTE) system, a single user supports multiplexing of a maximum of eight layers of orthogonal DMRS ports, and a DMRS occupies 24 resource elements (Res). Specifically, in frequency domain, DMRS ports may be mapped onto the zeroth, the first, the fifth, the sixth, the tenth, and the eleventh subcarriers in each resource block (RB) pair, and in time domain, DMRS ports may be mapped onto the fifth, the sixth, the twelfth, and the thirteenth symbols in each subframe, as shown in FIG. 1.
However, as people have increasingly high communication requirements such as a high rate, high reliability, and a low latency, modern communications systems will always face challenges of a larger capacity, wider coverage, and a lower latency. These requirements are also key requirements on a New Radio (NR) future network.
In a demodulation process at a receive end in the communications systems, compared with incoherent demodulation, coherent demodulation has better performance, and has a performance gain of approximately 3 dB. Therefore, the coherent demodulation is more widely used in the modern communications systems. However, modulation on each carrier in an orthogonal frequency-division multiplexing (OFDM) system is to suppress the carrier. Reference signals (RS), also referred to as pilot signals, are required during the coherent demodulation at the receive end. In an OFDM symbol, they are distributed on different resource units in two-dimensional time-frequency space, and have amplitudes and phases that are known. Likewise, in a MIMO system, each transmitting antenna (a virtual antenna or a physical antenna) has an independent data channel. Based on a predicted RS signal, a receiver performs channel estimation for each transmitting antenna, and restores sent data based on the estimation.
The channel estimation is a process in which a received signal is reconstructed to compensate for channel fading and noise. In this process, time-domain and frequency-domain changes of a channel are tracked by using RSs predicted by a transmitter and a receiver. For example, to implement data demodulation in a high-order multi-antenna system, an LTE-A system defines a demodulation reference signal (DMRS). The reference signal is used for demodulating uplink and downlink control channels and a data channel such as a physical downlink shared channel (PDSCH).
A same preprocessing manner is used for the DMRS and user data. Characteristics of the DMRS are as follows:
(1) The DMRS is user-specific. To be specific, a same precoding matrix is used for each piece of terminal data and a demodulation reference signal corresponding to the terminal data.
(2) From a perspective of a network side, DMRSs transmitted on layers are mutually orthogonal.
(3) The DMRS is usually used to support beamforming and precoding technologies, and therefore, is sent only on a scheduled resource block, where a quantity of sent DMRS ports is related to a quantity of data streams (or referred to as a quantity of layers). The DMRS ports are in one-to-one correspondence with antenna ports rather than a quantity of physical antennas. The quantity of DMRS ports is less than or equal to the quantity of physical antennas, and the two quantities are associated through layer mapping and precoding.
In a current standard, a maximum quantity of orthogonal data streams that can be supported by DMRSs used on a downlink is 8, resource overheads of each PRB pair are 24 REs, and the DMRSs are distributed in all PRBs in forms of block pilots. Each port (port) occupies 12 REs. In other words, densities of the ports are the same. In addition, a design of a DMRS sequence is determined based on the density of each port, and therefore, a length of the DMRS sequence is a fixed value.
However, New Radio (NR) supports more diverse scenarios, and therefore supports a plurality of configurations (patterns). For example, to adapt to data transmission in different frequency bands, multiplexing modes differ greatly. In addition, to further satisfy a larger-capacity transmission requirement, a maximum quantity of orthogonal data streams that can be supported by DMRSs on a data channel is greater than 8. For example, in the 3GPP RAN1 #88bis meeting, it was agreed that 12 orthogonal DMRS ports are supported.
Moreover, in the LTE system, all transceiver antennas have a very low dimension. Therefore, a multiple user (MU) dimension supported during MU matching is relatively low. For example, during MU scheduling, a maximum of two layers are allowed for a single user, and there are a total of four orthogonal layers. Compared with the LTE system, in a future network, four receive antennas may be necessary for future UE. In this case, an MU dimension changes.
During actual transmission, a base station needs to notify a terminal of information such as a quantity of layers that are allocated by the base station, a DMRS port number, a sequence configuration, and a multiplexing mode. In LTE, all of the information is indicated by using downlink control information (DCI). However, NR has supported a plurality of patterns, there are a plurality of variations in a quantity of ports, a multiplexing mode, and a mapping rule, and very high overheads are caused if the DCI-based indication manner in LTE is still used. Therefore, how to indicate a DMRS in NR is a technical problem that urgently needs to be resolved.